One of the currently popular forms of optical storage of information is the compact disk or CD. Digital information is stored in the form of high optical density marks or pits on an otherwise reflective background. In this format, the optical information is most often in the form of read only memory or ROM. Optical information is not usually recorded in real time but rather is produced by press molding. In a typical process, the optical recording substrate is first press molded with a master containing the digital information to be reproduced. The thus formed information is then overcoated with a reflective layer and then with an optional protective layer. In those areas having the deformations or pits, the optical density is higher than in those areas not having the deformations.
It is desirable to produce optical recording media which, when recorded in real time, produces a record that mimics the conventional CD on read out. In this manner, information can be added to the CD and the CD can be used on a conventional CD player. This adds additional complexity to the CD since a layer that can be recorded upon is interposed between the support and the reflecting layer.
One recently disclosed system of this type is the so called "Photo CD". In this system, conventional photographic film is first processed in a conventional manner. Then, the images from the film are digitized and the digital information is recorded in a CD readable form on an optical recording material. Images can then be played back on a conventional CD type player into a conventional television. Since a CD has a capacity for a number of digitized images that is greater than the typical roll of consumer film, it is anticipated that the user will want to add images to a preexisting CD. Thus the need for recordable, CD compatible optical recording material.
One important characteristic of the finished element is the durability of the surface of the element. As noted, the support is coated with a recording layer, a reflective layer and an optional protective layer. Since the readout of the element is through the support, characterizing information for the CD, such as logos, designs and printed information can be applied by screen printing to the protective layer.
The protective layer itself can be applied by a variety of means. Spin coating of a UV curable composition is common for many different types of optical elements. Workers have generally concentrated on finding coating compositions having desirable properties.
We have found that there are a number of problems associated with conventional overcoat layers and methods, particularly when applied to writable CD types of optical elements. In particular, it has not been possible to obtain layers of sufficient scratch resistance using a single spin coated layer. Such layers do not have sufficient thickness so as to provide the necessary protection. Spin coating multiple layers so as to build up thickness is possible but is economically not desirable. It has been suggested that thicker layers can be produced using screen printing methods but screen printing onto the reflective layer of the writable CD type of element produces unacceptable block error rates. In addition, there is a problem of providing adequate adhesion of the durability enhancing layer to the disk. This is complicated by the complex structure of the writable CD element since, compared to a conventional CD, there is an additional recording layer interposed between the durability enhancing layer and the support. Adhesion among all of these layers has been a problem. Thus, there is a continuing need for improved durability enhancing layer structures for writable CD materials.